1. Field of the Invention
The present invention relates to a film bulk acoustic resonator, and to a method of making the same.
2. Description of the Related Art
Portable telephones and other mobile telecommunications devices have spread far and wide during the past few years because of their handiness. This trend is still continuing, or even accelerating each time new models are put on the market. Accordingly, small wave filters used in these telecommunications devices are in great demand.
As is known, wave filters for portable telephones may be fabricated by utilizing surface acoustic wave (“SAW”) elements. In general, a SAW filter exhibits sharp cut-off characteristics and can be made light and small. Thus, SAW filters are widely used as RF (radio frequency) filters or IF (intermediate frequency) filters in portable telephones.
Typically, a SAW filter includes a piezoelectric substrate and a comb-teeth electrode formed on the substrate. In accordance with the alternating voltage applied to the comb-teeth electrode, elastic waves corresponding to certain frequency bands are produced in the surface of the piezoelectric substrate. A problem of a SAW filter is that the comb-teeth electrode may be distorted, and even broken, due to the elastic waves resulting from the application of high voltage. This drawback becomes conspicuous as the comb-teeth electrode is smaller in width, that is, as the filter is designed to deal with higher frequencies.
A filter for high-voltage use may be provided by utilizing film bulk acoustic resonators (referred to as “FBARs” hereinafter). An FBAR includes a substrate and a piezoelectric member held between upper and lower electrodes. The lower electrode is supported by the substrate, and this substrate is formed with a hollow portion under the lower electrode. When a voltage is applied between the upper and the lower electrodes, the piezoelectric member vibrates in its thickness direction, exhibiting its particular electric resonance characteristics. A band-pass-filter is constructed by arranging such FBARs in ladder form. The thus obtained FBAR filter is known to have excellent voltage-resistant properties. The hollow portion under the lower electrode improves the electromechanical coupling coefficient of the piezoelectric member, and widens the pass band of the filter.
Techniques to provide a hollow portion under a lower electrode are disclosed in JP-A-6(1994)-204776 and JP-A-2000-69594 for example. Specifically, JP-A-6-204776 teaches a hollow portion to be formed in a silicon wafer from below by anisotropic etching with the use of KOH solution. The hollow portion extends through the thickness of the wafer. On the other hand, JP-A-2000-69594 teaches using a sacrificial layer to fill in a recess formed in the surface of a substrate. More specifically, first, a recess is formed in the upper surface of the substrate. Then, a sacrificial layer is formed on the upper surface of the substrate in a manner such that part of the sacrificial layer is introduced into the recess. Thereafter, the sacrificial layer is removed by grinding, except for the recess-filling portion of the layer. Then, a lower electrode is formed on the substrate to cover the remnant of the sacrificial layer. Further, a piezoelectric member and an upper electrode are stacked on the lower electrode. Finally, the remnant of the sacrificial layer is removed, whereby a hollow portion appears under the lower electrode.
The above conventional techniques suffer from the following problems. Specifically, the silicon wafer of JP-A-6-204776 may be easily broken upon application of external force since the mechanical strength of the wafer is reduced by the hollow portion. Such fragility may require for extra care in dealing with the silicon wafer (e.g. in transporting the wafer from one working spot to another spot), and may lower the production yield rate. Another problem is caused by the etching process which renders the hollow portion to flare out in the downward direction. The inclination of the inner surface of the hollow portion may be about 55 degrees. With such a hollow portion varying in diameter in the thickness direction of the wafer, the respective resonators constructing a ladder-form filter tend to be unduly bulky. In the case of the technique disclosed in JP-A-2000-69594, several extra steps such as a recess-forming step, a sacrificial layer-forming step, a sacrificial layer-grinding step and a sacrificial layer-removing step are involved in the fabrication procedure. Thus, according to the teaching of JP-A-2000-69594, a resulting resonator is rather expensive due to the increased production costs.
In addition to the above-mentioned documents, JP-A-8(1996)-148968 also discloses an FBAR provided with a hollow portion under the lower electrode. The piezoelectric member is made of ceramic. However, this document does not specifically show a method of making the hollow portion nor a method of making an FBAR.